The present invention relates to electrochemichromic solutions and devices based thereon. Such solutions are well-known and are designed to either color or clear, depending on desired application, under the influence of applied voltage.
Such devices have been suggested for use as rearview mirrors in automobiles such that in night driving conditions, application of a voltage would darken a solution contained in a cell incorporated into the mirror (U.S. Pat. No. 3,280,701, Oct. 25, 1966). Similarly, it has been suggested that windows incorporating such cells could be darkened to block out sunlight, and then allowed to lighten again at night. Electrochemichromic cells have been used as display devices and have been suggested for use as antidazzle and fog-penetrating devices in conjunction with motor vehicle headlamps (British Patent Specification 328017, May 15, 1930).
U.S. Pat. No. 4,090,782 to Bredfeldt et al, U.S. Pat. No. 4,752,119 to Ueno et al (Jun. 1988), Chemical Abstract 86:196871c, 72-Electro. Chemistry, Vol. 86, 1977, I.V. Shelepin et al in Electrokhimya, 13(3), 404-408 (Mar. 1977), O. A. Ushakov et al, Electrokhimya, 14(2), 319-322 (Feb. 1978), U.S.S.R. Patent 566863 to Shelepin (Aug. 1977), U.S. Pat. No. 3,451,741 to Manos, European Patent Publication 240,226 published Oct. 7, 1987 to Byker, U.S. Pat. No. 4,902,108 to Byker, U.S. Pat. No. 3,806,229 to Schoot et al, U.S. Pat. No. 4,093,358 to Shattuck et al, European Patent Publication 0012419 published June 25, 1980 to Shattuck and U.S. Pat. No. 4,139,276 to Clecak et al all disclose electrochemichromic solutions of anodic and cathodic electrochromically coloring components which provide self-erasing, high color contrast, single compartment cells. Such anodic and cathodic coloring components comprise redox couples selected to exhibit the following reaction: ##STR1## The redox couple is selected such that the equilibrium position of the mixture thereof lies completely to the left of the equation. At rest potential, the anodically coloring reductant species RED.sub.1, and the cathodically coloring oxidant species OX.sub.2 are colorless. To cause a color change, voltage is applied and the normally colorless RED.sub.1 is anodically oxidized to its colored antipode OX.sub.1, while, simultaneously, OX.sub.2 is cathodically reduced to its colored antipode, RED.sub.2. These cathodic/anodic reactions occur preferentially at the electrodes which, in practical devices, are typically transparent conductive electrodes. Within the bulk of the solution, the redox potentials are such that when RED.sub.2 and OX.sub.1 come together, they revert to their lower energy form.
This means the applied potential need only suffice to drive the above reaction to the right. On removing the potential, the system reverts to its low energy state and the cell spontaneously self-erases.
Such redox pairs are placed in solution in an inert solvent. Typically, an electrolyte is also added. This solution is then placed into a relatively thin cell, between two conductive surfaces. In most applications, at least one of the conductive surfaces comprises a very thin layer of a transparent conductor such as indium tin oxide (ITO), doped tin oxide or doped zinc oxide deposited on a glass substrate so that the cell is transparent from at least one side. If the device is to be used in a mirror, the second surface is typically defined by a relatively thin layer of transparent conductor such as indium tin oxide, doped tin oxide or doped zinc oxide deposited on another glass substrate, which is silvered or aluminized or otherwise reflector coated on its opposite side. In the case of solar control windows, the second glass substrate would of course not be silvered on its opposite side so that when the redox pair is colorless, the window would be entirely transparent.
A known disadvantage of single-compartment, solution-phase, self-erasing electrochromic devices is exhibited whenever such a device is colored for an extended period of time, sometimes for as short as 60 seconds but more usually over several minutes or hours. When first bleached by removal of the electrical energy after prolonged operation or coloration, bleaching in the device is often non-uniform. Bands of color remain adjacent to the electrically conductive bus bars due to voltage gradient induced segregation, a phenomenon related to the depth of coloration which is a function of applied electrical potential. The resultant potential gradient induces diffusion of charged molecules resulting in increased concentration of colored species adjacent the bus bars.
Differences in solubilities between the colored and uncolored forms of any of the electrochromic species may also contribute to segregation. This form of segregation is particularly noticeable in electrochemichromic devices whose major plane is non-horizontal when in use such as is experienced with rearview mirrors on automobiles, wall mirrors, building windows, automotive front, rear and side windows and the like. In their colored form, either or both of the electrochemichromically active species may fully or partially come out of solution, and dependent on their density relative to that of the host solvent, may float or sink.
The effects of coloration segregation in electrochromic devices have both cosmetic and functional disadvantages. Bands of colors seen after extended coloration can be aesthetically displeasing in devices such as rearview mirrors, windows, office partitions, information displays and the like where users may question whether the device is damaged or working properly. In information display devices where regions of the device are colored while immediately adjacent regions remain uncolored, the functionality of such devices can be impaired because diffusion of colored molecules into adjacent uncolored regions reduces or eliminates lines of demarcation and thus information definition. While this can occur even during the period of prolonged operation, it is particularly evident upon first bleaching after such a period of extended use. To date, these segregation effects have limited the usefulness and commercial success of many electrochromic devices.
Another problem associated with electrochemichromic devices relates to consumer or user safety. Electrochemichromic solutions typically use chemicals of potential consumer or user risk from eye irritation, skin irritation, oral ingestion or the like. Also, these solutions are typically sandwiched between glass elements. Should these glass elements shatter upon impact in an accident, a consumer or user could be exposed to potential risk due to scattering of glass shards, splashing or spillage of electrochemichromic solution or the like. Also, even where the glass elements merely crack during normal usage, such that the electrochemichromic solution held therebetween simply oozes out to the exterior, a potential user contact hazard exists and, further, because such electrochemichromic solutions typically utilize organic solvents, surfaces such as automobile painted body work adjacent or in contact with the electrochemichromic device may suffer damage.
Yet another problem encountered in electrochemichromic devices relates to current leakage. When the electrochemichromic cell is colored by the application of voltage, the colored species OX.sub.1 and RED.sub.2 continually want to recombine and return to their equilibrium, colorless condition. The rate of recombination of the colored species OX.sub.1 and RED.sub.2 within the bulk of the solution is directly proportional to their diffusion coefficient in the solvent used. In order to compensate for the tendency of the colored species to recombine and go to the colorless equilibrium state, current must continually leak into the electrochemichromic solution via the conductive electrodes that typically sandwich said solution.
Because current must flow across the conductive surface of the transparent conductor used on at least one of the substrates that sandwich the electrochemichromic cell, and because these transparent conductors have finite sheet resistance, applied potential will be highest adjacent to the bus bar connector typically located at an edge perimeter and will be lowest near the center of the device as current passes across the conductive glass surface to color remote regions. Thus, if the leakage current is high and/or the sheet resistance of the transparent conductor is high, the potential drop that ensues across the transparent conductor itself results in a lower potential being applied to remote regions. Coloration is therefore non-uniform with the edge regions nearest the bus bar coloring deepest and the central regions coloring lightest.
If the leakage current is low due to a low rate of recombination of the colored species OX.sub.1 and RED.sub.2 within the bulk of the solution, deep coloring devices are facilitated and coloration uniformity is enhanced. However, bleach rate, which is the time taken to return from the full or partial colored state to the substantially clear, uncolored state, for low leakage current devices can be slow. Thus, it is advantageous when formulating an electrochemichromic solution to design the leakage current to achieve the commercially desired balance between depth of coloration, coloration uniformity and bleach rate. Also, it is desirable to reduce segregation effects by thickening the solution and to enhance safety and product protection by increasing solution viscosity. However, addition of prior art thickening agents such as acrylic polymers to the prior art solvents or the like, while effective in increasing solution viscosity such that anti-segregation and safety benefits are realized, has the disadvantage of lowering the leakage current of the selected formulation with a concomitant slowing of the device bleach rate. Thus, formulation to a desired device and safety performance is complicated.
Problems such as these have contributed to the failure of electrochemichromic solutions and devices based thereon to achieve the degree of commercial success which they potentially could achieve.